China is exceptionally rich in resources to develop applications of rare earth. In the currently ascertained rare earth resources around the world, 80% of the rare earth resources exist in China and the varieties are complete. In order to protect the resources and avoid the environmental problems caused by excessive development, China has began to manage and control the export of rare earth ore since 2009, but this action was protested by America, Japan, Europe and other countries. This reflects the preciousness of the rare earth resources and the necessary and the urgency in great development of deep processing of rare earth from another perspective. Directed to rich and distinctive rare earth resources in China, the optical materials which are the most distinctive rare earth elements and have been well accumulated in China are taken as the main development direction in the transition of Chinese rare earth industry to high-tech functional material industry, which per se also reflects the great industrial development requirements of China.
Organic electroluminescence is self-luminescent and has luminescent materials rich in colors for selection, and thus has the advantages of high efficiency, high brightness (>10,000 cd/m2), high contrast (>1000:1), wide color gamut (>100% NTSC), wide viewing angle (0-180°), fast response (microsecond grade) and the like in the properties of display and luminescence, and furthermore, light, thin (less than 1 mm) and flexible display is realized, and these performances exceed those of all the existing display technologies, so that the organic electroluminescence is generally acknowledged as the next generation of flat panel display technologies and lighting technologies.
Central luminescent ions of the rare earth complex can be divided into visible region strongly-luminescent rare earth ions, weakly-luminescent rare earth ions, rare earth ions with f-d radiation transitions and visible region non-luminescent rare earth ions. For example. Eu2+, Ce3+ and Tm3+ emit blue light. Eu3+ emits red light, Tb3+ emits green light, Sm3+ emits pink light, Dy3+ emits yellow light and Nd3+, Er3+ and Yb3+ emit near-infrared light. The radiation transitions of Tb3+ and Eu3+ fall within a visible light region, and during research of luminescent materials of the rare earth complex, the two types of ions attract the most attention, wherein a main emission peak of Tb3+ is positioned at about 545 nm and the color is very pure green; and the main emission peak of Eu3+ is positioned at about 613 nm, and the color is red with great eye sensitivity. The sensitized luminescence property of the rare earth can be applied to OLED display/lighting technologies in practices, biomedical test and anti-counterfeiting label printing, as well as infrared communication technologies. Since 1990, Kido team has firstly proved that β-diketone complexes of terbium can be used as luminescent materials for OLED devices. As they have narrow emission peaks and half-peak width of less than 10 nm, the chroma is saturated and bright, and the photo-quantum efficiency of rare earth organic luminescent materials is ultrahigh, the photo-quantum efficiency of reported solid europium complexes can achieve 85% (Coordination Chemistry Reviews, 2000, 196: 165), and the development of the rare earth organic luminescent materials re-attracts high attention of scientific community. The OLED devices of the europium complexes can obtain red light with saturated chroma; and the OLED devices of the terbium complexes can obtain green light with pure chroma. However, the efficiency and the service life of these devices fall far behind their theoretical expectations. The main reasons comprise poor film-forming ability of small molecular rare earth complexes, poor transmission performance of a current carrier and poor electrical, optical and thermal stability. As for other types of luminescent materials, such as small molecular organic luminescent materials, high polymer luminescent materials, complexes of precious metals, such as iridium, platinum and gold, and the like, although the emission peaks are wide, and the half-peak width is generally 80-100 nm, the color is dim in comparison with the rare earth luminescent materials; however, the efficiency and the service life of these luminescent materials have achieved the practical requirements.
The rare earth element needs 9-coordination to achieve saturated coordination; and simultaneously, as the rare earth metal ions have positive charges, ligands need to have negative charges to meet electrical neutrality. So far, the rare earth complex luminescent materials have adopted mixed ligands, e.g. 1,10-phenanthroline, β-diketone, and pyridine carboxylic acid compounds as ligands or for providing negative charges.
Up till now, novel rare earth luminescent complexes which are suitable for actual applications and even suitable for OLED display and lighting technologies have not been reported.